When most of us think of aquatic ecosystems, what usually comes to mind are oceans, rivers, bays, lakes, and streams. But there is another essential body of water, one not commonly known, that provides critical habitats for many woodland animals.

Vernal pools are similar to swamps or small ponds in appearance, but there’s one key difference: They fill and dry cyclically throughout the year. These temporary woodland ponds occur in shallow depressions and typically fill in the spring and dry out during the summer only to fill once again in the fall. Small in size, usually less than an acre, vernal pools are often surrounded by woodlands.

Several species of frogs, toads, salamanders, and numerous invertebrates use these pools as their primary breeding habitat, making their role in landscapes in the northeastern United States even greater than one might expect given their small size and temporary nature.

Vernal pools usually are at their deepest in the spring, which is where these pools get their name: vernal comes from the Latin “vernus,” meaning “belonging to spring.” They fill with rainwater, snow melt, and runoff from higher areas, and though small, they are literally teeming with life. The first warm rains of March and April set off mass migrations of frogs and salamanders from the surrounding woodlands into the pools, which provide a space for all sorts of plants, insects, and other animals to grow and thrive.

Take marbled salamanders, for instance. At summer’s end, many of the vernal pools are completely dry. By the end of September, prior to the onset of fall rains, hundreds of female marbled salamanders assemble and lay up to 200 eggs in depressions under logs, vegetation, and leaves in the lower areas of a vernal pools. The eggs are guarded until rains fill the low-lying areas, and the eggs hatch soon after coming into contact with water.

Fairy shrimp eggs that have been lying dormant in the dry mud also start hatching when these pools fill with water. Species such as wood frogs and spotted salamanders almost exclusively utilize vernal pools for breeding. Even mollusks, such as fingernail clams, can be found in vernal pools, surviving by remaining dormant in pool sediment during the dry season.

This wetland-then-drought cycle means that fish and other species that depend on permanent water cannot survive, providing an ideal habitat for the aquatic larvae of insects and amphibians. Any frog or salamander that lays its eggs in a vernal pool benefits by not having its offspring eaten by fish. These species would otherwise be challenged with competition or predation from larger aquatic species. The inhabitants of these vernal pools aren’t without predators, however. In April and May, snakes, turtles, birds, and mammals visit the vernal pools to feed on amphibians and their larvae. As the year progresses other species are drawn to vernal pools as well for food, water and shelter.

In addition to the providing a diverse ecosystem for wildlife, vernal pools help to gather and hold runoff from heavy rains, serving as storage tanks and settlement ponds for areas such as the Chesapeake Bay watershed. Without vernal pools, the runoff and silt load is increased and delivered directly into larger water sources.

Despite their vast importance, these wetland ecosystems are threatened. Because they are temporary, they are often not protected by wetlands laws. The study of these vernal pools is evolving, and ecologists are steadily increasing their understanding of these pools as a healthy habitat and breeding ground for many species.

This spring, you can see these vibrant ecosystems for yourself as you hike or bike through the forests of Maryland. These specialized, woodland wetlands can often be located by following the sounds of calling frogs. Take care not to disturb them, but by all means pause and look at the diversity of life that exists in these vernal pools.